Municipal waste is most commonly sent to landfills or incinerated. For many municipalities, landfills are preferred over incineration due to lower infrastructure cost and reduced concerns about immediate air pollution resulting from incineration. The heat resulting from incineration can be used to generate electricity.
Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, are a group of polyhalogenated compounds which are significant because they act as environmental pollutants. Dioxins are produced in small concentrations when organic material is burned in the presence of chlorine, whether the chlorine is present as chloride ions or as organochlorine compounds. Dioxins occur as by-products in the municipal incineration of chlorine-containing substances, such as PVC (polyvinyl chloride). In incineration, dioxins can also form in the atmosphere above the stack as the exhaust gases cool through a temperature window of 650° C. to 250° C. Reducing the quantity of dioxins can be achieved through rapid quenching of the exhaust gases through this 400 degree window. Thus, the problem of dioxin formation is avoided in conventional gasification systems.
Municipal waste gasification plants are not in widespread use, as is the case for incinerators. Gasification plants are commonly used to convert coal into fuel gas. In gasification, the waste is heated in the presence of reduced oxygen or air supply so that a synthesis gas of hydrogen and carbon monoxide is produced.
The synthesis gas can contain tars and a secondary gasifier can be used to convert tars to synthesis gas. Before combusting the synthesis gas, it may be scrubbed to remove contaminants. The synthesis gas can then be used as a fuel supply for power generation or other energy needs. In PCT publication WO2008/044216 dated Apr. 17, 2008, there is described by the Applicant of the present application a waste disposal system that gasifies small batches of waste to produce a clean synthesis fuel gas. A Primary Gasifier chamber is heated by a combustion fuel source with a small amount of introduced air to produce a crude synthesis gas (containing tars) that is then passed to a secondary gasifier chamber that is heated by a combustion or electric fuel source to an optimum gasification temperature to produce synthesis gas essentially free of tars. The synthesis gas can be quenched and cleaned of other contaminants, such as acid gases, and then it can replace a conventional fuel source driving the gasification process as it becomes available. The synthesis gas is decontaminated and the combustion gases resulting from the synthesis gas are clean and can pass directly to the atmosphere as flue gas. When the batch process is completed, a small amount of carbonaceous residue, typically less than 10% of the original volume, and a small amount of various filtered contaminants remain.